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Abstract

The rate of phosphorus release from sediments, and the degree to which alum reduces phosphorus release from sediments, was investigated in Spring Lake during the summer of 2003. Triplicate cores were sampled from 4 sites in the lake, and exposed to one of four treatments in the laboratory: 1) oxygen/alum; 2) oxygen/ no alum; 3) nitrogen/alum; or 4) nitrogen/no alum. Both soluble reactive phosphorus (SRP) and total phosphorus (TP) release rates were virtually undetectable in the alum treatments (both with and without oxygen). Low, but detectable, release rates were measured in the oxygen/no alum treatment. The highest release rates were measured in the nitrogen/no alum treatments; these rates translated to an internal load that ranged between 2.8 (low range) to 7.7 (high range) tons/year when extrapolated to a whole-lake basis. Internal phosphorus loads were approximately double that of previously estimated external phosphorus loads, and accounted for between 56 and 66% of the total phosphorus load to Spring Lake.

Uncertainties in our estimates are discussed, including spatial heterogeneity of sediment type, calculation of percent anoxia in Spring Lake, extrapolation of laboratory release rates to the whole lake, and external load estimates. We conclude that internal loading is a significant source of phosphorus to Spring Lake, and that alum is a potentially effective means of reducing this source. However, it is unknown how long an alum treatment would remain effective in this system. We recommend: 1) additional laboratory studies to obtain an estimate of how long an alum treatment would remain effective; 2) a pilot field study; and 3) continued efforts to reduce external loading to Spring Lake.